1. Field of the Invention
This invention relates to the preparation of aromatic amines and more particularly pertains to an improved process for the preparation of polymethylene polyarylpolyamine mixtures by the acid catalyzed condensation reaction of an aromatic primary amine and formaldehyde.
2. Description of the Prior Art
The condensation reaction of aromatic primary amines, such as aniline, substituted anilines, etc., and formaldehyde carried out in the presence of an acid-containing material catalyst at elevated temperatures is well known and has been employed for many years in procedures for the preparation of methylene-bridged polyarylpolyamine mixtures useful as epoxy resin curing agents, urethane cross-linkers, precursors for corresponding polyisocyanates, and the like. These known processes are amply described in the literature and many patents, for example U.S. Pat. Nos. 2,683,730; 2,950,263; 3,012,008; 3,344,162; 3,362,979; 3,277,173; 3,496,229 and Belgian Patent 648,787, to name a few.
Generally speaking, procedures for preparing polymethylene polyarylpolyamine mixtures include mixing and reacting an excess of an aromatic primary amine, such as aniline, substituted anilines, and the like, with formaldehyde at an elevated temperature above about 105.degree. C in the presence of a catalytic amount of a strong mineral acid, e.g., hydrochloric acid. The reaction is usually complete in about 4 to 5 hours and provides a resulting reaction product of polymethylene polyarylpolyamine containing a diamine portion in admixture with higher functionality higher molecular weight methylene-bridged polyarylpolyamines. The diamine portion of the reaction product mixture can be substantially controlled by varying several reaction conditions, such as the molar ratio of aromatic primary amine to formaldehyde. Moreover, as known, the positional isomer content of the diamine portion of the reaction product, namely the 2,2'-, 2,4'-, and 4,4'-, diamine isomers can be varied by varying reaction conditions and employing different types of acid-containing materials as the catalyst.
However, known procedures employing strong mineral acids as catalysts in such condensation reaction procedures not only suffer from the difficulty of requiring long reaction times but also require the employment of corrosive-resistant reaction and handling equipment, inasmuch as such acids are highly corrosive. Moreover, the employment of such mineral acids require neutralization of the crude condensation reaction product mixtures with a basic material such as caustic to produce the desired polymethylene polyarylpolyamine mixtures. Neutralization brings on the additional difficulties of by-product removal and disposal.
Several procedures are known in the art which do not suffer from the aforementioned disadvantages. For example, U.S. Pat. No. 3,362,979 to Bentley describes a procedure for the preparation of methylene-bridged polyphenylpolyamine mixtures by carrying out the condensation reaction of aniline and formaldehyde in the presence of solid acid-siliceous catalysts such as silica-alumina cracking catalysts. As disclosed, the employment of solid acid-siliceous catalysts such as silica-alumina eliminates the necessity of employing corrosive-resistant processing apparatus and the necessity of neutralizing the reaction product mixture to provide the desired methylene-bridged polyphenylpolyamine product mixtures. The patent further teaches that the employment of such solid catalysts results in the production of polyphenylpolyamine mixtures having diamine portions that contain greater amounts of the 2,4'- isomer than obtained by conventional strong mineral acid catalyzed condensation reaction procedures. Yet such solid acid siliceous catalyst procedures suffer from other disadvantages such as slow condensation reaction rates and difficulty in separating the solid catalyst materials from the resulting reaction product mixtures. These solid catalysts are employed in finely powdered form and separating them from the crude condensation reaction product mixtures, such as by filtration, has been found to be difficult.
U.S. Pat. No. 3,496,229 to Powers et al. also teaches a process for preparing polymethylene polyarylpolyamine mixtures that does not suffer from the aforementioned disadvantages of requiring the use of corrosive-resistant processing apparatus and the neutralization of the resulting reaction product. Patentees teach a process for the aforementioned condensation reaction of an aromatic primary amine and formaldehyde which is carried out in the presence of a low-level synergistic catalyst system which is comprised of a mixture of an acid which has a pKa value (measured at 25.degree. C in water) of 1.5 to 5 and a catalytic amount of neutral salt of a metal and a nonmetal. More particularly, the acid is employed in an amount sufficient to provide a molar ratio of the aromatic amine reactant to the acid of about 100 to 1 to about 200,000 to 1 and the neutral salt is present in an amount in the range of 0.01 to 10%. However, the procedure suffers from the same disadvantage as the aforementioned procedure employing solid acid-siliceous catalysts in that the neutral salts of the catalyst system must be removed from the resulting reaction product mixture, such as by filtration. The required employment of the neutral salt in the co-catalyst system and subsequent necessity of removal from the reaction product are added expense and time consuming requirements of such procedures.
I have now discovered a process for the preparation of methylene-bridged polyarylpolyamine mixtures by the catalyzed condensation reaction of an aromatic primary amine and formaldehyde which does not suffer from any of the aforementioned disadvantages associated with the aforementioned procedures. The process of the present invention can be carried out in less expensive reaction and handling apparatus and does not require neutralization of the reaction product or filtration of any catalyst material, particularly when low concentrations of catalyst are used. The inventive process may be conducted at relatively high temperatures for relatively short periods of time which greatly improves the economics for preparing polymethylene polyarylpolyamine mixtures.